About the air- and water-stable copper(I) dicyanamide: synthesis, crystal structure, vibrational spectra and DSC/TG analysis of Cu[N(CN)₂]

2.1 Synthesis

All manipulations were performed under normal atmospheric conditions. An aqueous Cu⁺ solution was obtained in situ by dissolving Cu[NO₃]₂·2.5 H₂O (Sigma Aldrich, ACS Grade 98%, St. Louis, MO, USA) and an excess of K₂[S₂O₅] (Fisher, certified, Fair Lawn, NJ, USA). Upon adding stoichiometric amounts of Na[dca] (Alfa Aesar, 96%, powder, Ward Hill, MA, USA), from the resulting apple-green solution nearly colorless thin platelets of Cu[dca] precipitated. The compound was isolated with the help of a Büchner funnel, washed with ice-cold ethanol and dried. Green powder samples of Cu[N(CN)₂]₂ (≡Cu[dca]₂) were obtained by blending stoichiometric amounts of aqueous Cu[NO₃]₂·2.5 H₂O and Na[dca]. Cu[CN] (Aldrich, 99.99%, Milwaukee, WI, USA) was used as received from Aldrich. Both dicyanamides precipitate quantitatively.

All compounds are stable towards normal atmosphere. Cu[dca] adopts a light yellowish tint after some days of atmospheric exposure. After several weeks the tint becomes greenish, but the X-ray pattern and the vibrational spectra remain unchanged.

2.2 Crystal structure determination

A powder X-ray diffraction (PXRD) pattern of the solid phase of Cu[dca] for structure determination was collected at room temperature on a laboratory powder diffractometer in Debye-Scherrer geometry (Stadi-P Diffractometer (Stoe), CuK_α1 radiation from a Ge(111) Johann-type monochromator, Mythen 1 K detector (Dectris)). The sample was sealed in a 0.5 mm diameter borosilicate glass capillary (Hilgenberg glass, no. 14), which was spun during the measurement. Data were taken for 12 h in steps of 0.015° (2θ) from 2 to 92° (2θ). The program Topas 4.2 [8] was used to determine and to refine the crystal structure. Indexing of the phase was carried out by an iterative use of the singular value decomposition (LSI) [9] leading to a centered orthorhombic unit cell with lattice parameters of a=356.28(3), b=611.10(9) and c=1525.87(10) pm. C2₁ma (no. 36), Cm2a (no. 40) and Cmcm (no. 63) were estimated from the observed extinction rules as the most probable space groups of which the latter was confirmed by the crystal structure refinement.

The peak profiles and the precise lattice parameters were determined by a Pawley whole powder pattern fit [10] by applying the fundamental parameter approach of Topas [8]. The background was modeled by employing Chebyshev polynomials of the 6^th order. The refinement converged quickly. The apparent anisotropic peak width could successfully be modeled by the phenomenological anisotropic microstrain model of Stephens [11]. The crystal structure of the solid phase was solved by applying the global optimization method of simulated annealing (SA) in real space as implemented in TOPAS [12]. After a few minutes, the positions of all atoms were found. For the final Rietveld refinement [13], all profile and lattice parameters were released iteratively and all atomic positions were subjected to free unconstrained refinement. As the two clearly distinguishable types of C–N bond lengths were slightly out of the expected range, slack soft constraints of 116 and 131 pm were introduced. This did not change the overall weighted agreement factors. The final refinement lead to a reasonable R_wp value of 4.8% and an R_Bragg value of 1.2%. The results, atomic coordinates and selected bond distances and angles are given in Tables 1–3 the Rietveld refinement fit is shown in Fig. 1.

Fig. 1:

Results of the Rietveld refinement for Cu[dca].

Further details of the crystal structure investigation may be obtained from FIZ Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (Fax: (+49)7247-808-666; E-mail: crysdata@fiz-karlsruhe.de) on quoting the deposition number CSD-432067 for orthorhombic Cu[dca].

2.3 Raman and IR spectroscopy

Powders of Cu[dca] and Cu[dca]₂ were sealed in thin-walled glass capillaries. Raman spectroscopic investigations were performed on a microscope laser Raman spectrometer (Jobin Yvon, Unterhaching, Germany, 4 mW, equipped with a HeNe laser with an excitation line at λ=632.817 nm, 50× magnification, 8×240 s accumulation time).

The IR spectra of Cu[dca] and Cu[dca]₂ were obtained with a Bruker AFS 66 FT-IR instrument (Karlsruhe, Germany) with the KBr pellet technique (2 mg product being ground together with 400 mg dried KBr).

The combined IR and Raman spectra of Cu[dca] and Cu[dca]₂ are displayed in Fig. 2, the exact frequencies and their assigned modes are shown in Table 1.

Fig. 2:

Vibrational spectra of Cu[dca] (top) and Cu[dca]₂ (bottom). On the vertical axis: Raman intensities or IR transmission, respectively, in arbitrary units. Wavenumbers are given in cm⁻¹.

2.4 DSC/TG measurements

A total of 8.277 mg of Cu[dca], 7.417 mg Cu[dca]₂ and 5.895 mg Cu[CN], respectively, were placed on an DSC/TG pan made out of corundum. This set-up was introduced into a Netzsch STA 449C instrument (Selb, Germany) under a constant stream of pure argon. After flushing the material at room temperature for 10 min, the mass balanced back to the respective numbers given above. Each sample was heated with 10 K min⁻¹ up to 800°C (Figs. 3–5).

Fig. 3:

Results of the DSC/TG measurement for Cu[CN].

Fig. 4:

Results of the DSC/TG measurement for Cu[dca].

Fig. 5:

Results of the DSC/TG measurement for Cu[dca]₂.

3.1 Optical spectra

The frequencies obtained from the IR and Raman spectra of Cu[dca] and Cu[dca]₂ compare well to the vibrational frequencies reported in the literature for Cu[dca] and Cu[dca]₂ [6] (Table 4) and confirm the presence of the [dca]⁻ anion.

3.2 DSC/TG measurements

Fluctuations at temperatures below 100°C are artifacts due to a machine malfunction. Cu[dca] shows a mass loss of almost 2% probably due adhering water incorporated from moisture out of the air. At 319°C an exothermic reaction can be observed. For Na[dca] [14], K[dca] and Rb[dca] [15] a large and broad exothermic peak between 300 and 400°C was observed for each compound indicating the trimerization of the [dca]⁻ anions ([N≡C–N–C≡N]⁻) to the cyclic [C₆N₉]³⁻ moiety. Above this temperature, the mass loss is becoming more pronounced showing several as-yet not assigned exo- and endothermic peaks, before at temperatures >700°C an endothermic peak and the lowest observed mass (50% of the water-free sample) indicate the final melting of copper metal. The leftover of the DSC/TG consisted of copper-red flakes and a coppery-red coated crucible. The red material dissolved in nitric acid setting free brown fumes and leaving a green-colored acid residue. This interpretation is in accordance with the calculated 49% residue expected if copper remains and nitrogen (N₂) and dicyan (N≡C–C≡N) were set free.

The DSC/TG measurement on Cu[dca]₂ shows analogous results: the artifact below 100°C, the exothermic trimerization occurring at 269°C, several not yet assigned exo- and endothermic peaks with a high mass loss (70%), an endothermic peak indicating melting at the lowest indicated mass and a residue of 30% (calculated: 29%) consisting of copper-red flakes, which dissolved in nitric acid setting free brown fumes and leaving a green-colored acid residue.

Both DSC/TG measurements are corroborated by the measurements performed on copper(I) cyanide Cu[CN]. A phase transition can be observed at 292°C indicated by a small exothermic peak; slow decomposition starts at about 400°C, while melting at 477°C shows rapid decomposition. At the end of the thermolysis at about 600°C, 70% of the initial mass remains, indicating copper as residual material (calculated: 71%), which is proven by the appearance (copper-red coating of the crucible) and its chemical reaction towards nitric acid (see above).

3.3 Crystal structure

The copper(I) dicyanamide Cu[dca] contains planar boomerang-shaped dicyanamide anions [(N(CN)₂]⁻ already well investigated and characterized, e.g. in the crystal structures of Li[N(CN)₂] [16] and the two polymorphs of Ag[N(CN)₂] [17], [18]. In Cu[dca], the dicyanamide anion exhibits C_2v symmetry according to [(N2)C(N1)C(N2)]⁻ with distances d(N1–C)=131 pm, d(C–N2)=116 pm and ∡(C–N1–C)=128°. The short C≡N2 distance indicates a triple bond of the cyanide part of the [N(CN)₂]⁻ anion (Fig. 6, top), while the distances N1–C show markedly larger values (131 pm). According to Fig. 6, the Cu⁺ cation is predominantly coordinated in a linear fashion by two terminal nitrogen atoms (N2) of two different [dca]⁻ anions with d(Cu–N2)=179 pm. All bond lengths and angles are in expected ranges, when compared with similar dicyanamides with monovalent cations [16], [17], [18]. In the crystal structure of Cu[dca] the individual dicyanamide anions are vertex-connected via linear and symmetric bridges N2–Cu–N2 (∡(N2–Cu–N2)=180°) with bond lengths d(Cu–N2)=179 pm (2×) forming planar zig-zag chains along [001] (Fig. 7, top). This finding is in contrast to the three-dimensional network found in Li[N(CN)₂] [16], where Li⁺ shows four- and six-fold coordination with nitrogen atoms of dicyanamide anions. For Cu[dca] and for the orthorhombically crystallizing polymorph of Ag[dca] [17], [18], infinite chains ∞1{M[N(CN)2]}2/2v (M=Cu⁺ and Ag⁺; v≡vertex-connecting) are formed consisting of a predominantly linear two-fold coordination of the coinage-metal(I) cations by dicyanamide anions (Fig. 7). Additionally, secondary interactions of the monovalent cations with negatively polarized nitrogen atoms of adjacent chains are evident in Cu[dca] and in both forms of Ag[dca]. For copper(I) dicyanamide four additional contacts of Cu⁺ cations of one chain to only terminal nitrogen atoms of four neighboring chains d(Cu···N2′)=324 pm (4×) (Fig. 6, top) lead to a strictly planar arrangement of the zig-zag chains along [100] with isotactic stacking of adjacent chains along [001] as replicated by the C-centering of the orthorhombic unit cell (Fig. 7, top). In the trigonal polymorph of silver dicyanamide, t-Ag[dca] (P3₁21), the same (2+4) coordination for Ag⁺ cations (Fig. 6, mid) with two strong N–Ag–N intra-chain bonds (d(Ag–N)=211, pm 2×) and four weak inter-chain interactions (d(Ag···N)=297–305 pm, 4×) to exclusively terminal nitrogen atoms of adjacent chains lead once more to zig-zag chains, but these form twisted helices along the trigonal c axis (Fig. 7, mid). In orthorhombic Ag[dca] (Pnma) on the other hand, Ag⁺ shows also a (2+4) coordination (d(Ag–N)=213 pm, 2×, and d(Ag···N)=288–297 pm, 4×), but in this case the four secondary contacts to adjacent chains are built up to two terminal nitrogen atoms and two central nitrogen atoms of the dicyanamide units (Fig. 6, bottom). Therefore, the ideal C_2v symmetry is broken and an arrangement of planar chains with mutual syndiotactic stacking (described by a glide plane n perpendicular to the a axis) along [010] results (Fig. 7, bottom).

Fig. 6:

Decorations of the dicyanamide anions with coinage-metal(I) cations in the crystal structures of orthorhombic Cu[dca] (top), trigonal Ag[dca] (mid) and orthorhombic Ag[dca] (bottom).

Fig. 7:

Infinite chains ∞1{M[N(CN)2]}2/2v (M=Cu and Ag) in the crystal structures of orthorhombic Cu[dca] (top), trigonal Ag[dca] (mid) and orthorhombic Ag[dca] (bottom).